When upgrading from anything prior to 9, I always upgrade to 9 first, then to MCD 4.2 (and beyond)... I have had phones not boot due to firmware issues. Older firmware doesn't have a large enough boot partition for the firmware in MCD 4.2, loading into 9 and letting all the phones flash, then upgrading resolves the issues.
But to the OPs problem, x-man is correct, you must get a new license when going from 8 to 9 or any major version change, and SWA is the only way to get that.